Joy Butterfly
Joy Butterflies are special butterflies filled with blue colour. They appear as you fuse Kinstones with people. They have different abilities, like growing your digging speed, etc.
Joy Butterflies are small and pretty slow that they are easier to catch. Just walk over them and you have the butterfly.
Quote from The Legend of Zelda: The Minish Cap Nayru figurine: "A Rare butterfly said to bring happiness to those who catch it. If you see one on your journeys, try to grab it!"